Longistylin A from Cajanus cajan (L.) Millsp. disturbs glycerophospholipid metabolism and cytokinin biosynthesis of Nocardia seriolae

ETHNOPHARMACOLOGICAL RELEVANCE

Nocardiosis is an uncommon infectious disease that bears certain similarities to tuberculosis, with a continuous increase in its incidence and a poor prognosis. In traditional Chinese medicine, the leaves of Cajanus cajan (L.) Millsp. are employed to treat wounds, malaria, coughs, and abdominal pain.

AIM OF THE STUDY

In this study, we investigated the effects and mechanisms of longistylin A (LGA), a natural stilbene isolated from C. cajan, as a potential antibiotic against nocardiosis.

MATERIALS AND METHODS

LGA was isolated from the leaves of C. cajan and assessed using a minimum bactericidal concentration (MBC) determination against Nocardia seriolae. Multi-omics analysis encompassing genes, proteins, and metabolites was conducted to investigate the impact of LGA treatment on N. seriolae. Additionally, quantitative analysis of 40 cytokinins in N. seriolae mycelium was performed to assess the specific effects of LGA treatment on cytokinin levels. Cryo-scanning electron microscopy was utilized to examine morphological changes induced by LGA treatment, particularly in the presence of exogenous trans-zeatin-O-glucoside (tZOG). The therapeutic effect of LGA was investigated by feeding N. seriolae-infected largemouth bass.

RESULTS

LGA exhibited significant efficacy against N. seriolae, with MBC value of 2.56 μg/mL. Multi-omics analysis revealed that LGA disrupted glycerophospholipid metabolism and hormone biosynthesis by notably reducing the expression of glycerol-3-phosphate dehydrogenase and calmodulin-like protein. Treatment with LGA markedly disrupted 12 distinct cytokinins in N. seriolae mycelium. Additionally, the addition of exogenous tZOG counteracted the inhibitory effects of LGA on filamentous growth, resulting in mycelial elongation and branching. Furthermore, LGA treatment improved the survival rate of largemouth bass infected with N. seriolae.

CONCLUSIONS

We found for the first time that LGA from C. cajan exhibited significant efficacy against N. seriolae by interfering with glycerophospholipid metabolism and cytokinin biosynthesis.

The role of phospholipid molecular species in determining the physical properties of yeast membranes

In most eukaryotes, including Saccharomyces cerevisiae, glycerophospholipids are the main membrane lipid constituents. Besides serving as general membrane ‘building blocks’, glycerophospholipids play an important role in determining the physical properties of the membrane, which are crucial for proper membrane function. To ensure optimal physical properties, membrane glycerophospholipid composition and synthesis are tightly regulated. This review will summarize our current knowledge of factors and processes determining the membrane glycerophospholipid composition of the reference eukaryote S. cerevisiae at the level of molecular species. Extrapolating from relevant model membrane data, we also discuss how modulation of the molecular species composition can regulate membrane physical properties.

Fanconi Anemia Mesenchymal Stromal Cells-Derived Glycerophospholipids Skew Hematopoietic Stem Cell Differentiation Through Toll-Like Receptor Signaling

Fanconi anemia (FA) patients develop bone marrow (BM) failure or leukemia. One standard care for these devastating complications is hematopoietic stem cell transplantation. We identified a group of mesenchymal stromal cells (MSCs)-derived metabolites, glycerophospholipids, and their endogenous inhibitor, 5-(tetradecyloxy)-2-furoic acid (TOFA), as regulators of donor hematopoietic stem and progenitor cells. We provided two pieces of evidence that TOFA could improve hematopoiesis-supporting function of FA MSCs: (a) limiting-dilution cobblestone area-forming cell assay revealed that TOFA significantly increased cobblestone colonies in Fanca-/- or Fancd2-/- cocultures compared to untreated cocultures. (b) Competitive repopulating assay using output cells collected from cocultures showed that TOFA greatly alleviated the abnormal expansion of the donor myeloid (CD45.2+Gr1+Mac1+) compartment in both peripheral blood and BM of recipient mice transplanted with cells from Fanca-/- or Fancd2-/- cocultures. Furthermore, mechanistic studies identified Tlr4 signaling as the responsible pathway mediating the effect of glycerophospholipids. Thus, targeting glycerophospholipid biosynthesis in FA MSCs could be a therapeutic strategy to improve hematopoiesis and stem cell transplantation.

Influence of Development and Dietary Phospholipid Content and Composition on Intestinal Transcriptome of Atlantic Salmon (Salmo salar)

The inclusion of intact phospholipids in the diet is essential during larval development and can improve culture performance of many fish species. The effects of supplementation of dietary phospholipid from marine (krill) or plant (soy lecithin) sources were investigated in Atlantic salmon, Salmo salar. First feeding fry were fed diets containing either krill oil or soybean lecithin supplying phospholipid at 2.6%, 3.2%, 3.6% and 4.2% of diet. Fish were sampled at ~ 2.5 g (~1,990°day post fertilization, dpf) and ~10 g (2,850°dpf). By comparison of the intestinal transcriptome in specifically chosen contrasts, it was determined that by 2,850°dpf fish possessed a profile that resembled that of mature and differentiated intestinal cell types with a number of changes specific to glycerophospholipid metabolism. It was previously shown that intact phospholipids and particularly phosphatidylcholine are essential during larval development and that this requirement is associated with the inability of enterocytes in young fry to endogenously synthesize sufficient phospholipid for the efficient export of dietary lipid. In the immature phase (~1,990°dpf), the dietary phospholipid content as well as its class composition impacted on several biochemical and morphological parameters including growth, but these differences were not associated with differences in intestinal transcriptomes. The results of this study have made an important contribution to our understanding of the mechanisms associated with lipid transport and phospholipid biosynthesis in early life stages of fish.

Relevance of dietary glycerophospholipids and sphingolipids to human health

Glycerophospholipids and sphingolipids participate in a variety of indispensable metabolic, neurological, and intracellular signaling processes. In this didactic paper we review the biological roles of phospholipids and try to unravel the precise nature of their putative healthful activities. We conclude that the biological actions of phospholipids activities potentially be nutraceutically exploited in the adjunct therapy of widely diffused pathologies such as neurodegeneration or the metabolic syndrome. As phospholipids can be recovered from inexpensive sources such as food processing by-products, ad-hoc investigation is warranted.

Large-scaled metabolic profiling of human dermal fibroblasts derived from pseudoxanthoma elasticum patients and healthy controls

Mutations in the ABC transporter ABCC6 were recently identified as cause of Pseudoxanthoma elasticum (PXE), a rare genetic disorder characterized by progressive mineralization of elastic fibers. We used an untargeted metabolic approach to identify biochemical differences between human dermal fibroblasts from healthy controls and PXE patients in an attempt to find a link between ABCC6 deficiency, cellular metabolic alterations and disease pathogenesis. 358 compounds were identified by mass spectrometry covering lipids, amino acids, peptides, carbohydrates, nucleotides, vitamins and cofactors, xenobiotics and energy metabolites. We found substantial differences in glycerophospholipid composition, leucine dipeptides, and polypeptides as well as alterations in pantothenate and guanine metabolism to be significantly associated with PXE pathogenesis. These findings can be linked to extracellular matrix remodeling and increased oxidative stress, which reflect characteristic hallmarks of PXE. Our study could facilitate a better understanding of biochemical pathways involved in soft tissue mineralization.

Regulatory networks, genes and glycerophospholipid biosynthesis pathway in schistosomiasis: a systems biology view for pharmacological intervention

Understanding network topology through embracing the global dynamical regulation of genes in an active state space rather than traditional one-gene-one trait approach facilitates the rational drug development process. Schistosomiasis, a neglected tropical disease, has glycerophospholipids as abundant molecules present on its surface. Lack of effective clinical solutions to treat pathogens encourages us to carry out systems-level studies that could contribute to the development of an effective therapy. Development of a strategy for identifying drug targets by combined genome-scale metabolic network and essentiality analyses through in silico approaches provides tantalizing opportunity to investigate the role of protein/substrate metabolism. A genome-scale metabolic network model reconstruction represents choline-phosphate cytidyltransferase as the rate limiting enzyme and regulates the rate of phosphatidylcholine (PC) biosynthesis. The uptake of choline was regulated by choline concentration, promoting the regulation of phosphocholine synthesis. In Schistosoma, the change in developmental stage could result from the availability of choline, hampering its developmental cycle. There are no structural reports for this protein. In order to inhibit the activity of choline-phosphate cytidyltransferase (CCT), it was modeled by homology modeling using 1COZ as the template from Bacillus subtilis. The transition-state stabilization and catalytic residues were mapped as 'HXGH' and 'RTEGISTT' motif. CCT catalyzes the formation of CDP-choline from phosphocholine in which nucleotidyltransferase adds CTP to phosphocholine. The presence of phosphocholine permits the parasite to survive in an immunologically hostile environment. This feature endeavors development of an inhibitor specific for cytidyltransferase in Schistosoma. Flavonolignans were used to inhibit this activity in which hydnowightin showed the highest affinity as compared to miltefosine.

Monitoring phosphatidic acid formation in intact phosphatidylcholine bilayers upon phospholipase D catalysis

We have monitored the production of the negatively charged lipid, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidic acid acid (POPA), in supported lipid bilayers via the enzymatic hydrolysis of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (PC), a zwitterionic lipid. Experiments were performed with phospholipase D (PLD) in a Ca(2+) dependent fashion. The strategy for doing this involved using membrane-bound streptavidin as a biomarker for the charge on the membrane. The focusing position of streptavidin in electrophoretic-electroosmotic focusing (EEF) experiments was monitored via a fluorescent tag on this protein. The negative charge increased during these experiments due to the formation of POPA lipids. This caused the focusing position of streptavidin to migrate toward the negatively charged electrode. With the use of a calibration curve, the amount of POPA generated during this assay could be read out from the intact membrane, an objective that has been otherwise difficult to achieve because of the lack of unique chromophores on PA lipids. On the basis of these results, other enzymatic reactions involving the change in membrane charge could also be monitored in a similar way. This would include phosphorylation, dephosphorylation, lipid biosynthesis, and additional phospholipase reactions.

The caveolin-1 scaffolding domain peptide decreases phosphatidylglycerol levels and inhibits calcium-induced differentiation in mouse keratinocytes

Phospholipase D2 (PLD2) has been found localized in low-density caveolin-rich membrane microdomains. Our previous study suggested that PLD2 and aquaporin 3 (AQP3) interact in these domains to inhibit keratinocyte proliferation and promote differentiation by cooperating to produce phosphatidylglycerol. To examine the effect of membrane microdomain localization on the PLD2/AQP3 signaling module and keratinocyte proliferation and differentiation, we treated mouse keratinocytes with 3 µM cell-permeable caveolin-1 scaffolding domain peptide or a negative control peptide and stimulated cell differentiation using a moderately elevated extracellular calcium concentration (125 uM) to maximally promote differentiation and phosphatidylglycerol production. Cell proliferation, differentiation, total PLD activity, phosphatidylglycerol levels, and AQP3 activity were monitored. The caveolin-1 scaffolding domain peptide itself had no effect on phosphatidylglycerol levels or keratinocyte proliferation or differentiation but prevented the changes induced by a moderately elevated calcium concentration, whereas a negative control did not. The caveolin-1 scaffolding domain peptide had little effect on total PLD activity or glycerol uptake (AQP3 activity). We conclude that the caveolin-1 scaffolding domain peptide disrupts the functional association between AQP3 and PLD2 and prevents both the inhibited proliferation and the stimulated differentiation in response to elevated extracellular calcium levels. The interaction of caveolin-1 and PLD2 is indirect (i.e., lipid mediated); together with the proliferation-promoting effects of caveolin-1 knockout on epidermal keratinocytes, we propose that the caveolin-1 scaffolding domain pepetide exerts a dominant-negative effect on caveolin-1 to alter lipid rafts in these cells.

The ether lipid precursor hexadecylglycerol causes major changes in the lipidome of HEp-2 cells

The ether-lipid precursor sn-1-O-hexadecylglycerol (HG) can be used to compensate for early metabolic defects in ether-lipid biosynthesis. To investigate a possible metabolic link between ether-linked phospholipids and the rest of the cellular lipidome, we incubated HEp-2 cells with HG. Mass spectrometry analysis revealed major changes in the lipidome of HG-treated cells compared to that of untreated cells or cells treated with palmitin, a control substance for HG containing an acyl group instead of the ether group. We present quantitative data for a total of 154 species from 17 lipid classes. These species are those constituting more than 2% of their lipid class for most lipid classes, but more than 1% for the ether lipids and glycosphingolipids. In addition to the expected ability of HG to increase the levels of ether-linked glycerophospholipids with 16 carbon atoms in the sn-1 position, this precursor also decreased the amounts of glycosphingolipids and increased the amounts of ceramide, phosphatidylinositol and lysophosphatidylinositol. However, incubation with palmitin, the fatty acyl analogue of HG, also increased the amounts of ceramide and phosphatidylinositols. Thus, changes in these lipid classes were not ether lipid-dependent. No major effects were observed for the other lipid classes, and cellular functions such as growth and endocytosis were unaffected. The data presented clearly demonstrate the importance of performing detailed quantitative lipidomic studies to reveal how the metabolism of ether-linked glycerophospholipids is coupled to that of glycosphingolipids and ester-linked glycerophospholipids, especially phosphatidylinositols.

The glycerophospholipid inventory of Pseudomonas putida is conserved between strains and enables growth condition-related alterations

Microorganisms, such as Pseudomonas putida, utilize specific physical properties of cellular membrane constituents, mainly glycerophospholipids, to (re-)adjust the membrane barrier to environmental stresses. Building a basis for membrane composition/function studies, we inventoried the glycerophospholipids of different Pseudomonas and challenged membranes of growing cells with n-butanol. Using a new high-resolution liquid chromatography/mass spectrometry (LC/MS) method, 127 glycerophospholipid species [e.g. phosphatidylethanolamine PE(32:1)] with up to five fatty acid combinations were detected. The glycerophospholipid inventory consists of 305 distinct glycerophospholipids [e.g. PE(16:0/16:1)], thereof 14 lyso-glycerophospholipids, revealing conserved compositions within the four investigated pseudomonads P. putida KT2440, DOT-T1E, S12 and Pseudomonas sp. strain VLB120. Furthermore, we addressed the influence of environmental conditions on the glycerophospholipid composition of Pseudomonas via long-time exposure to the sublethal n-butanol concentration of 1% (v/v), focusing on: (i) relative amounts of glycerophospholipid species, (ii) glycerophospholipid head group composition, (iii) fatty acid chain length, (iv) degree of saturation and (v) cis/trans isomerization of unsaturated fatty acids. Observed alterations consist of changing head group compositions and for the solvent-sensitive strain KT2440 diminished fatty acid saturation degrees. Minor changes in the glycerophospholipid composition of the solvent-tolerant strains P. putida S12 and Pseudomonas sp. VLB120 suggest different strategies of the investigated Pseudomonas to maintain the barrier function of cellular membranes.

Putative N-acylphosphatidylethanolamine synthase from Arabidopsis thaliana is a lysoglycerophospholipid acyltransferase

AT1G78690, a gene found in Arabidopsis thaliana, has been reported to encode a N-acyltransferase that transfers an acyl chain from acyl-CoA to the headgroup of phosphatidylethanolamine (PE) to form N-acylphosphatidylethanolamine (N-acyl-PE). Our investigation suggests that At1g78690p is not a PE-dependent N-acyltransferase but is instead a lysoglycerophospholipid O-acyltransferase. We overexpressed AT1G78690 in Escherichia coli, extracted the cellular lipids, and identified the accumulating glycerophospholipid as acylphosphatidylglycerol (acyl-PG). Electrospray ionization quadrupole time-of-flight mass spectrometry (ESI-MS) analysis yielded [M - H](-) ions, corresponding by exact mass to acyl-PG rather than N-acyl-PE. Collision-induced dissociation mass spectrometry (MS/MS) yielded product ions consistent with acyl-PG. In addition, in vitro enzyme assays using both (32)P- and (14)C-radiolabeled substrates showed that AT1G78690 acylates 1-acyllysophosphatidylethanolamine (1-acyllyso-PE) and 1-acyllysophosphatidylglycerol (1-acyllyso-PG), but not PE or phosphatidylglycerol (PG), to form a diacylated product that co-migrates with PE and PG, respectively. We analyzed the diacylated product formed by AT1G78690 using a combination of base hydrolysis, phospholipase D treatment, ESI-MS, and MS/MS to show that AT1G78690 acylates the sn-2-position of 1-acyllyso-PE and 1-acyllyso-PG.

Two closely related genes of Arabidopsis encode plastidial cytidinediphosphate diacylglycerol synthases essential for photoautotrophic growth

Cytidinediphosphate diacylglycerol synthase (CDS) catalyzes the formation of cytidinediphosphate diacylglycerol, an essential precursor of anionic phosphoglycerolipids like phosphatidylglycerol or -inositol. In plant cells, CDS isozymes are located in plastids, mitochondria, and microsomes. Here, we show that these isozymes are encoded by five genes in Arabidopsis (Arabidopsis thaliana). Alternative translation initiation or alternative splicing of CDS2 and CDS4 transcripts can result in up to 10 isoforms. Most of the cDNAs encoding the various plant isoforms were functionally expressed in yeast and rescued the nonviable phenotype of the mutant strain lacking CDS activity. The closely related genes CDS4 and CDS5 were found to encode plastidial isozymes with similar catalytic properties. Inactivation of both genes was required to obtain Arabidopsis mutant lines with a visible phenotype, suggesting that the genes have redundant functions. Analysis of these Arabidopsis mutants provided further independent evidence for the importance of plastidial phosphatidylglycerol for structure and function of thylakoid membranes and, hence, for photoautotrophic growth.

Rapid activation of specific phospholipase(s) D by cytokinin in Amaranthus assay system

The suggested link between intracellular cytokinin signaling and phospholipase D (PLD, EC 3.1.4.4.) activity (Romanov et al. 2000, 2002) was investigated. The activity of PLD in the early period of cytokinin action was studied in vivo in derooted Amaranthus caudatus seedlings, using the level of phosphatidylbutanol production as a measure of PLD activity. Rapid activation of phosphatidylbutanol synthesis was demonstrated as early as within 5 min of cytokinin administration. Neomycin, a known phosphatidylinositol-4,5-bisphosphate (PIP(2)) antagonist, strongly repressed both physiological cytokinin effect and cytokinin-dependent PLD activation. N-acylethanolamine (NAE 12), an inhibitor of alpha-class PLD, did not influence significantly cytokinin effect on Amaranthus seedlings. Together, results suggest the involvement of PIP(2)-dependent non-class alpha-PLD in the molecular mechanism of cytokinin action.

SLC1 and SLC4 encode partially redundant acyl-coenzyme A 1-acylglycerol-3-phosphate O-acyltransferases of budding yeast

Phosphatidic acid is the intermediate, from which all glycerophospholipids are synthesized. In yeast, it is generated from lysophosphatidic acid, which is acylated by Slc1p, an sn-2-specific, acyl-coenzyme A-dependent 1-acylglycerol-3-phosphate O-acyltransferase. Deletion of SLC1 is not lethal and does not eliminate all microsomal 1-acylglycerol-3-phosphate O-acyltransferase activity, suggesting that an additional enzyme may exist. Here we show that SLC4 (Yor175c), a gene of hitherto unknown function, encodes a second 1-acyl-sn-glycerol-3-phosphate acyltransferase. SLC4 harbors a membrane-bound O-acyltransferase motif and down-regulation of SLC4 strongly reduces 1-acyl-sn-glycerol-3-phosphate acyltransferase activity in microsomes from slc1Delta cells. The simultaneous deletion of SLC1 and SLC4 is lethal. Mass spectrometric analysis of lipids from slc1Delta and slc4Delta cells demonstrates that in vivo Slc1p and Slc4p generate almost the same glycerophospholipid profile. Microsomes from slc1Delta and slc4Delta cells incubated with [14C]oleoyl-coenzyme A in the absence of lysophosphatidic acid and without CTP still incorporate the label into glycerophospholipids, indicating that Slc1p and Slc4p can also use endogenous lysoglycerophospholipids as substrates. However, the lipid profiles generated by microsomes from slc1Delta and slc4Delta cells are different, and this suggests that Slc1p and Slc4p have a different substrate specificity or have access to different lyso-glycerophospholipid substrates because of a different subcellular location. Indeed, affinity-purified Slc1p displays Mg2+-dependent acyltransferase activity not only toward lysophosphatidic acid but also lyso forms of phosphatidylserine and phosphatidylinositol. Thus, Slc1p and Slc4p may not only be active as 1-acylglycerol-3-phosphate O-acyltransferases but also be involved in fatty acid exchange at the sn-2-position of mature glycerophospholipids.

alpha(1)-Adrenergic receptor stimulation of cell motility requires phospholipase D-mediated extracellular signal-regulated kinase activation

Phospholipase D is suspected to play a role in tumorigenesis, and the inhibition of phospholipase D has been associated with changes in several cellular events including invasion and migration. We report here that the specific alpha(1)-adrenergic receptor agonist, phenylepherine, signals to a growth factor pathway in a manner that requires phospholipase D activity in CCL39 fibroblasts. Phenylepherine increased extracellular signal-regulated kinase phosphorylation eightfold and promoted stress fiber formation threefold. Stress fiber formation was blocked when extracellular signal-regulated kinase activation was inhibited. Stimulation of CCL39 fibroblasts by phenylepherine increased the rate of wound healing fourfold in a wounding assay, while treatment with the MEK inhibitor, PD98059 reduced the closure of phenylepherine-induced wound healing to control levels. Addition of 1-butanol but not 2-butanol inhibited extracellular signal-regulated kinase activation by phenylepherine, presumably by blocking the formation of phosphatidic acid. Exogenously added cell permeable phosphatidic acid increased extracellular signal-regulated kinase activation in a time- and dose-dependent manner as well as stimulated the formation of stress fibers. 1-butanol also significantly inhibited the ability of phenylepherine to stimulate stress fiber formation and wound healing. Taken together, these results indicate a novel role for phospholipase D in the activation of the extracellular signal-regulated kinase growth factor pathway to stimulate early cellular events induced by phenylepherine.

Lipid profiling reveals glycerophospholipid remodeling in zymosan-stimulated macrophages

Comprehensive lipid profiling by mass spectrometry provides comparative data on the relative distribution of individual glycerophospholipids within each of the major classes. Application of this method to the analysis of glycerophospholipid remodeling in murine primary resident peritoneal macrophages (RPMs) during zymosan phagocytosis reveals significant decreases in the levels of every major arachidonic acid (20:4)-containing species of phosphatidylcholine (GPCho) and in selected 20:4-containing phosphatidylinositol (GPIns) and phosphatidylglycerol (GPGro) species. No net changes in 20:4-containing phosphatidylethanolamine (GPEtn) species were detected. Pretreatment of RPMs with LPS resulted in subtle changes in the magnitude and kinetics of the response but had no effect on the overall pattern of zymosan-induced glycerophospholipid remodeling. Inhibition of prostaglandin (PG) synthesis with indomethacin reduced the magnitude of the changes in 20:4-containing diacyl but not alkyl acyl species. Blockade of 20:4 reacylation with thimerosal had no effect on the magnitude of the zymosan-induced changes in GPCho, GPIns, or GPGro species but revealed decreases in the level of alkyl acyl GEtn species. RAW264.7 cells contain much lower levels of phospholipid 20:4 than do RPMs and synthesize PGs poorly in response to zymosan. Pretreatment with granulocyte-macrophage colony stimulating factor, lipopolysaccharide, and interferon-gamma substantially increased the extent of 20:4 mobilization and PG synthesis in these cells. However, under conditions of maximal zymosan-dependent PG synthesis, the only glycerophospholipid that exhibited a significant change was a 20:4-containing plasmenyl GPEtn. These results suggest that GPCho is the major ultimate source of 20:4 that is mobilized in zymosan-stimulated RPMs but that 20:4 mobilization may involve the intermediate turnover of alkyl acyl GPEtn species.

A Single Enzyme Catalyzes Both Platelet-activating Factor Production and Membrane Biogenesis of Inflammatory Cells

Platelet-activating factor (PAF) is a potent proinflammatory lipid mediator eliciting a variety of cellular functions. Lipid mediators, including PAF are produced from membrane phospholipids by enzymatic cascades. Although a G protein-coupled PAF receptor and degradation enzymes have been cloned and characterized, the PAF biosynthetic enzyme, aceyl-CoA:lyso-PAF acetyltransferase, has not been identified. Here, we cloned lyso-PAF acetyltransferase, which is critical in stimulus-dependent formation of PAF. The enzyme is a 60-kDa microsomal protein with three putative membrane-spanning domains. The enzyme was induced by bacterial endotoxin (lipopolysaccharide), which was suppressed by dexamethasone treatment. Surprisingly, the enzyme catalyzed not only biosynthesis of PAF from lyso-PAF but also incorporation of arachidonoyl-CoA to produce PAF precursor membrane glycerophospholipids (lysophosphatidylcholine acyltransferase activity). Under resting conditions, the enzyme prefers arachidonoyl-CoA and contributes to membrane biogenesis. Upon acute inflammatory stimulation with lipopolysaccharide, the activated enzyme utilizes acetyl-CoA more efficiently and produces PAF. Thus, our findings provide a novel concept that a single enzyme catalyzes membrane biogenesis of inflammatory cells while producing a prophlogistic mediator in response to external stimuli.

Metabolic Profiling of Glycerophospholipid Synthesis in Fibroblasts Loaded with Free Cholesterol and Modified Low Density Lipoproteins

Currently, the detailed regulation of major pathways of glycerophospholipid synthesis upon cholesterol loading is largely unknown. Therefore, a detailed lipid metabolic profiling using stable isotope-labeled choline, ethanolamine, and serine was performed by quantitative electrospray ionization tandem mass spectrometry (ESI-MS/MS) in free cholesterol (FC), oxidized (Ox-LDL) and enzymatically modified LDL (E-LDL)-loaded primary human skin fibroblasts. As previously described, an adaptive induction of phosphatidylcholine (PC) synthesis via CDP-choline was found upon FC loading. In contrast to PC, CDP-ethanolamine-mediated phosphatidylethanolamine (PE) synthesis was inhibited by FC incubation. Furthermore, FC induced a shift toward polyunsaturated PE and PC species, which was mediated primarily by PE biosynthesis but not PE remodeling, whereas PC species were shifted mainly by fatty acid (FA) remodeling of existing PC. Modified lipoprotein incubation revealed rather different effects on glycerophospholipid synthesis. E-LDL greatly enhanced PC synthesis, whereas Ox-LDL did not change PC synthesis. Addition of different free FAs (FFA) with and without FC coincubation, as major components of E-LDL, clearly indicated an incorporation of FFA into newly synthesized PC and PE species as well as FFA as important driving force for PC synthesis. Because FC and FFA are known to affect lipid membrane properties including membrane curvature, these data support that CTP:phosphocholine cytidylyl-transferase activity and consequently PC synthesis are regulated by modulation of membrane characteristics at the cellular level. In conclusion, the application of high throughput metabolic profiling of major glycerophospholipid pathways by ESI-MS/MS is a powerful tool to unravel mechanisms underlying the regulation of cellular lipid metabolism.

Rhythms of glycerophospholipid synthesis in retinal inner nuclear layer cells

The present study demonstrates that the biosynthesis of phospholipids in the inner nuclear layer cells of the chicken retina displays daily rhythms under constant illumination conditions. The vertebrate retina contains circadian oscillators and photoreceptors (PRCs) that temporally regulate its own physiology and synchronize the whole organism to the daily environmental changes. We have previously reported that chicken photoreceptors and retinal ganglion cells (RGCs) present significant daily variations in their phospholipid biosynthesis under constant illumination conditions. Herein, we demonstrate that cell preparations highly enriched in inner nuclear layer cells also exhibit a circadian-regulated phospholipid labeling after the in vivo administration of [(32)P]phosphate or [(3)H]glycerol both in animals maintained under constant darkness or light for at least 48h. In constant darkness, there was a significant incorporation of both precursors into phospholipids with the highest levels of labeling around midday and dusk. In constant light, the labeling of (32)P-phospholipids was also significantly higher during the day and early night whereas the incorporation of [(3)H]glycerol into phospholipids, that indicates de novo biosynthesis, was greater during the day but probably reflecting a higher precursor availability at those phases. We also measured the in vitro activity of phosphatidate phosphohydrolase and diacylglycerol lipase in preparations obtained from the dark condition. The two enzymes exhibited the highest activity levels late in the day. When we assessed the in vitro incorporation of [(14)C]oleate into different lysophospholipids from samples collected at different phases in constant darkness, reaction catalyzed by lysophospholipid acyltransferases II, labeling showed a complex pattern of daily activity. Taken together, these results demonstrate that the biosynthesis of phospholipids in cells of the chicken retinal inner nuclear layer exhibits a daily rhythmicity under constant illumination conditions, which is controlled by a circadian clock.

Role of dihydroxyacetonephosphate acyltransferase in the biosynthesis of plasmalogens and nonether glycerolipids

The variant CHO-K1 cell line, NRel-4, is unable to synthesize plasmalogens because of a severe reduction in dihydroxyacetonephosphate acyltransferase (DHAPAT) activity (Nagan, N., A. K. Hajra, L. K. Larkins, P. Lazarow, P. E. Purdue, W. B. Rizzo, and R. A. Zoeller. 1998. Isolation of a Chinese hamster fibroblast variant defective in dihydroxyacetonephosphate acyltransferase activity and plasmalogen biosynthesis: use of a novel two-step selection protocol. Biochem. J. 332: 273-279). Northern analysis demonstrated that the loss of this activity was attributable to a severe reduction in mRNA levels for DHAPAT. Transfection of NRel-4 cells with a plasmid bearing the human DHAPAT cDNA recovered DHAPAT activity and plasmalogen biosynthesis. Examination of clonal isolates from the transfected population showed that recovery of as little as 10% of wild-type DHAPAT activity restored plasmalogen levels to 55% of normal, whereas in one isolate, NRel-4.15, which overexpressed DHAPAT activity by 6-fold over wild-type cells, plasmalogen levels were returned only to wild-type values. Although the rate of plasmenylethanolamine biosynthesis was restored in NRel-4.15, the biosynthesis of nonether glycerolipids was either decreased or unaffected, suggesting that peroxisomal DHAPAT does not normally contribute to nonether glycerolipid biosynthesis. These data demonstrate that a defect in the gene that codes for peroxisomal DHAPAT is the primary lesion in the NRel-4 cell line and that the peroxisomal DHAPAT is essential for the biosynthesis of plasmalogens in animal cells.

Up-regulation of phospholipase Cgamma1 and phospholipase D during the differentiation of human monocytes to dendritic cells

Phospholipase C (PLC)gamma and phospholipase D (PLD) play pivotal roles in the signal transduction required for various cellular responses, including cell proliferation and differentiation. Dendritic cells (DCs), which are professional antigen-presenting cells, can be generated from human monocytes by stimulating the cells with granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin 4 (IL-4). We investigated whether PLCgamma and PLD expression levels can be changed during the differentiation of the human monocytes into DCs. The enzymatic activity and protein level of PLC gamma1 were significantly increased in the human monocyte-derived DCs by GM-CSF/IL-4, but the protein levels of PLC gamma2 were unaltered. Moreover, the enzymatic activity and protein level of PLD1b and PLD2 were up-regulated during the differentiation of human monocytes to DCs, but those of PLD1a were not changed. A higher phagocytic activity of DCs was found to be correlated with the up-regulations of PLCgamma1 and PLD, and the phagocytic activity of DCs was inhibited by a PLC-specific inhibitor (U73122) and by a phosphatidic acid acceptor (n-butanol), but to be increased by phosphatidic acid. Thus, suggesting that PLC and PLD participate in the process. This study suggests that the up-regulations of PLCgamma1 and PLD are accompanied by the differentiation of monocytes into DCs, which results in increased phagocytic activity.

Atrial Natriuretic Peptide Effects on Intracellular pH Changes and ROS Production in HEPG2 Cells: Role of p38 MAPK and Phospholipase D

AIMS

The present study was performed to evaluate Atrial Natriuretic Peptide (ANP) effects on intracellular pH, phospholipase D and ROS production and the possible relationship among them in HepG2 cells. Cancer extracellular microenvironment is more acidic than normal tissues and the activation of NHE-1, the only system able to regulate pHi homeostasis in this condition, can represent an important event in cell proliferation and malignant transformation.

METHODS

The ANP effects on pHi were evaluated by fluorescence spectrometry. The effects on p38 MAPK and ROS production were evaluated by immunoblots and analysis of DCF-DA fluorescence, respectively. RT-PCR analysis and Western blotting were used to determine the ANP effect on mRNA NHE-1 expression and protein levels. PLD-catalyzed conversion of phosphatidylcholine to phosphatydilethanol (PetOH), in the presence of ethanol, was monitored by thin layer chromatography.

RESULTS

A significant pHi decrease was observed in ANP-treated HepG2 cells and this effect was paralleled by the enhancement of PLD activity and ROS production. The ANP effect on pHi was coupled to an increased p38 MAPK phosphorylation and a down-regulation of mRNA NHE-1 expression and protein levels. Moreover, the relationship between PLD and ROS production was demonstrated by calphostin-c, a potent inhibitor of PLD. At the same time, all assessed ANP-effects were mediated by NPR-C receptors.

CONCLUSION

Our results indicate that ANP recruits a signal pathway associated with p38 MAPK, NHE-1 and PLD responsible for ROS production, suggesting a possible role for ANP as novel modulator of ROS generation in HepG2 cells.

Contribution of different biosynthetic pathways to species selectivity of aminoglycerophospholipids assembled into mitochondrial membranes of the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, three pathways lead to the formation of cellular phosphatidylethanolamine (PtdEtn), namely the mitochondrial conversion of phosphatidylserine (PtdSer) to PtdEtn catalyzed by phosphatidylserine decarboxylase 1 (Psd1p), the equivalent reaction catalyzed by phosphatidylserine decarboxylase 2 (Psd2p) in the Golgi, and the CDP-ethanolamine branch of the so-called Kennedy pathway which is located to the microsomal fraction. To investigate the contributions of these three pathways to the cellular pattern of PtdEtn species (fatty acid composition) we subjected lipids of wild-type and yeast mutant strains with distinct defects in the respective pathways to mass spectrometric analysis. We also analyzed species of PtdSer and phosphatidylcholine (PtdCho) of these strains because formation of the three aminoglycerophospholipids is linked through their biosynthetic route. We demonstrate that all three pathways involved in PtdEtn synthesis exhibit a preference for the formation of C34:2 and C32:2 species resulting in a high degree of unsaturation in total cellular PtdEtn. In PtdSer, the ratio of unsaturated to saturated fatty acids is much lower than in PtdEtn, suggesting a high species selectivity of PtdSer decarboxylases. Finally, PtdCho is characterized by its higher ratio of C16 to C18 fatty acids compared to PtdSer and PtdEtn. In contrast to biosynthetic steps, import of all three aminoglycerophospholipids into mitochondria of wild-type and mutant cells is not highly specific with respect to species transported. Thus, the species pattern of aminoglycerophospholipids in mitochondria is mainly the result of enzyme specificities, but not of translocation processes involved. Our results support a model that suggests equilibrium transport of aminoglycerophospholipids between mitochondria and microsomes based on membrane contact between the two compartments.

The E5 protein of the human papillomavirus type 16 modulates composition and dynamics of membrane lipids in keratinocytes

The E5 protein of the human papillomavirus type 16 is a small protein found associated to membranes, mainly in the Golgi apparatus, and expressed in the early stages of viral infection. Its expression modifies the cell response towards growth factors and stress exposures, and also blocks the surface expression of MHC molecules. A global explanation for these multiple effects is hitherto not available. Here we present data showing that the expression of HPV16-E5 increases the amount of free cholesterol readily extractable from the plasma membrane, without altering the total cholesterol content. In addition, HPV16-E5 modifies the composition of the cell membranes, increasing the synthesis rate of phosphatidylcholine and phosphatidylserine, while diminishing that of phosphatidylglycerol. We propose that these changes in the lipid composition of the membrane are the central effect of HPV16-E5 on the cell. The multiple and apparently disconnected effects of HPV16-E5 on tyrosine-kinase receptors, induction of the apoptosis and impairment of MHC trafficking could follow the initial alteration on the membrane composition.

Lipid metabolism in cultured lichen photobionts with different phosphorus status

Lipid metabolism was studied in different photobiont species from lichens by following incorporation of radiolabel from [1-14C]acetate. In four algal photobionts, Coccomyxa mucigena, C. peltigera variolosae, Trebouxia aggregata, T. erici, polar lipids were mainly (73-90%) labelled while triacylglycerols were the most highly labelled non-polar lipid class. A rhamnose-containing lipid was found in two Coccomyxa species, representing about 11% of the polar lipids of C. mucigena. All the major algal glycosyl- and phospho-glycerides were labelled with monogalactosyldiacylglycerol and phosphatidylglycerol, respectively, being the main labelled lipids in the polar lipid classes. The photobionts were grown in media differing in their phosphate content by one hundred-fold. Low phosphate levels caused only a small decrease in the proportion of phosphoglyceride labelling--mainly in phosphatidylglycerol. However, total lipid labelling was reduced (by 83.3 and 76.6% in two Coccomyxa spp. and 62.1 and 27% in two Trebouxia spp.) for the green algae. By comparison, variations in phosphate availability had no significant effect on a Nostoc sp. Examination of the algal species by electron microscopy revealed phosphorus-containing granules. This reserve of phosphorus explains why the algal photobionts were able to maintain the proportion of phosphoglyceride labelling well and may be an important adaptive mechanism for lichens.

Degradation of phosphatidylethanol counteracts the apparent phospholipase D-mediated formation in heart and other organs

Phosphatidylalcohols, such as phosphatidylethanol (PEth), are formed from phosphatidylcholine in the presence of a primary alcohol (e.g., ethanol). This 'transphosphatidylation' reaction is used as specific phospholipase D (PLD) assay. Accumulation of PEth in tissues is recognized as a reliable measure of PLD activity, as PEth is allegedly metabolically stable. The general validity of this assumption was reinvestigated in isolated rat heart, small intestine and brain slices. The half-times of 3H-PEth degradation (labelled with 3H-myristic acid and preformed by ethanol exposure for 30 min) were about 1 h in heart and small intestine, but 17 h in brain. As the formation of PEth is superimposed by simultaneous degradation, a mathematical model was established to calculate the differences between 'true' and 'apparent' PEth formation. As expected, this difference was relevant in heart and intestine, but not in brain tissue. For example, ischemia in the perfused heart for 30 min reversibly blocked PEth degradation and seemingly enhanced PEth formation; the block was reversed by ischemic preconditioning (IPC) and by pretreatment with diazoxide, an opener of mitochondrial K(ATP) channels. In conclusion, PEth degradation in heart was energy-dependent and rapid, which, when ignored, may lead to misinterpretation of PEth values with respect to PLD activity.

Participation of phospholipase D and alpha/beta-protein kinase C in growth factor-induced signalling in C3H10T1/2 fibroblasts

We have studied phospholipase D (PLD) activation in relation to protein kinase C (PKC) and the involvement of PLD in extracellularly regulated kinase 1 (MAPK) (ERK1) activation and c-fos mRNA expression in C3H/10T1/2 (Cl8) fibroblasts. In these cells, the PLD activity was significantly increased by porcine platelet-derived growth factor (PDGF-BB), phorbol 12-myristate 13-acetate (PMA), and epidermal growth factor (EGF). PLD activation by PDGF-BB and PMA, but not EGF, was inhibited in Cl8 cells expressing the HAbetaC2-1 peptide (Cl8 HAbetaC2-1 cells), with a sequence (betaC2-1) shown to bind receptor for activated C kinase 1 (RACK1) and inhibit c-PKC-mediated cell functions [Science 268 (1995) 247]. A role of alpha-PKC in PLD activation is further underscored by co-immunoprecipitation of alpha-PKC with PLD1 and PLD2 in non-stimulated as well as PMA- and PDGF-BB-stimulated Cl8 cells. However, only PKC in PLD1 precipitates was activated by these agonists, while the PKC in the PLD2 precipitates was constitutively activated. The c-fos mRNA levels in Cl8 cells increased more than 30-fold in response to either PDGF-BB, EGF, or PMA. Approximately 60% inhibition of this increase in c-fos mRNA levels was observed in Cl8 HAbetaC2-1 cells. Formation of phosphatidylbutanol (PtdBut) at the expense of phosphatidic acid (PtdH) in the presence of n-butanol inhibited ERK1 activation and c-fos mRNA expression in PDGF-BB-treated Cl8 cells. ERK activation by PMA was unaffected by n-butanol in Cl8 cells but almost abolished by n-butanol in Cl8 HAbetaC2-1 cells, showing that ERK activation by PMA is heavily dependent on PKC and PLD1. In contrast, ERK activation by EGF in both cell types was not sensitive to n-butanol. These results indicate (1) a role of a functional interaction between the RACK1 scaffolding protein and a alphaPKC-PLD complex for achieving full PLD activity in PDGF-BB- and PMA-stimulated Cl8 cells; (2) PLD-mediated PtdH formation is needed for optimal ERK1 activation by PDGF-BB and maximal increase in c-fos mRNA expression. These findings place PLD as an important component in PDGF-BB- and PMA-stimulated intracellular signalling leading to gene activation in Cl8 cells, while EGF does not require PLD.

Nod factor and elicitors activate different phospholipid signaling pathways in suspension-cultured alfalfa cells

Lipo-chitooligosaccharides (Nod factors) are produced by symbiotic Rhizobium sp. bacteria to elicit Nod responses on their legume hosts. One of the earliest responses is the formation of phosphatidic acid (PA), a novel second messenger in plant cells. Remarkably, pathogens have also been reported to trigger the formation of PA in nonlegume plants. To investigate how host plants can distinguish between symbionts and pathogens, the effects of Nod factor and elicitors (chitotetraose and xylanase) on the formation of PA were investigated in suspension-cultured alfalfa (Medicago sativa) cells. Theoretically, PA can be synthesized via two signaling pathways, i.e. via phospholipase D (PLD) and via phospholipase C in combination with diacylglycerol (DAG) kinase. Therefore, a strategy involving differential radiolabeling with [(32)P]orthophosphate was used to determine the contribution of each pathway to PA formation. In support, PLD activity was specifically measured by using the ability of the enzyme to transfer the phosphatidyl group of its substrate to a primary alcohol. In practice, Nod factor, chitotetraose, and xylanase induced the formation of PA and its phosphorylated product DAG pyrophosphate within 2 min of treatment. However, whereas phospholipase C and DAG kinase were activated during treatment with all three different compounds, PLD was only activated by Nod factor. No evidence was obtained for the activation of phospholipase A(2).

Selenium deficiency alters the formation of eicosanoids and signal transduction in rat lymphocytes

Previous reports have shown that selenium (Se) nutrition alters the lipoxygenase pathway and mitogenic responses in bovine lymphocytes. In order to further understand how Se may alter lymphocyte function, we examined the effects of Se nutrition on arachidonic acid (AA) metabolism and phospholipase D (PLD) activation. Lymphocytes were isolated from the lymph nodes of rats fed either Se-deficient diet (-Se) or Se-supplemented diet (+Se) for 12 weeks. Our results revealed that calcium ionophore A23187-stimulated lymphocytes derived from -Se rats produced significantly less prostaglandins (PGs) than those obtained from +Se rats. Phospholipase D (PLD) activation by 12-O-tetradecanoylphorbol-13-acetate (TPA) was significantly lower in lymphocytes obtained from -Se rats when compared to cells from +Se rats. Furthermore, the addition of PGE2, PGD2 or PGF2alpha to suspended lymphocytes from -Se rats significantly enhanced PLD activity. The effects of TPA and PGE2 on PLD activation were additive. However, the addition of PGE2 abolished the significant difference in PLD activation between -Se and +Se cells observed in response to TPA alone. Based on these results, we postulate that dietary Se status plays an important role in the regulation of AA metabolism that subsequently affects PLD activation.

Regulation of sucrose and starch synthesis in wheat (Triticum aestivum L.) leaves: role of fructose 2,6-bisphosphate

Fructose 2,6-bisphosphate (F26BP) is a competitive inhibitor of the cytosolic fructose 1,6-bisphosphatase (cytFBPase, EC 3.1.3.11). In spinach (Spinacia oleracea L.) leaves it is a significant component of the complex regulatory network that co-ordinates rates of photosynthesis, sucrose synthesis and starch synthesis. However the role of F26BP has only been studied in plants that predominantly store starch in their leaves and its role in other species is not clear. This paper examines the significance of F26BP in the regulation of photosynthetic carbon metabolism in the intact leaves of wheat (Triticum aestivum L.), a plant that accumulates predominantly sucrose. The approach taken was to vary rates of photosynthesis and then correlate measurements of F26BP and a range of other metabolites with rates of carbohydrate synthesis obtained from (14)CO(2)-feeding experiments performed under physiological conditions. It was found that: (i) Amounts of 3-phosphoglycerate and fructose-6-phosphate are correlated with the amount of F26BP. (ii) F26BP is involved in inhibiting cytFBPase at low light and low CO(2), but other factors, for example triose-phosphate, must also be involved. (iii) Amounts of both F26BP and substrate are involved in co-ordinating rates of photosynthesis and sucrose synthesis, but the relative importance of these depends on the conditions. (iv) Amounts of F26BP do not correlate with the partitioning of fixed carbon between sucrose and starch. Together these data suggest that the amount of F26BP in wheat is regulated by mechanisms similar to those in spinach, and that the metabolite is one of the factors involved in co-ordinating sucrose synthesis and photosynthesis. However F26BP does not appear to be involved in regulating the partitioning of fixed carbon between sucrose and starch in wheat under the experimental conditions examined.

Phosphatidylethanol in post-mortem blood as a marker of previous heavy drinking

Phosphatidylethanol (PEth) is an ethanol-phospholipid adduct, formed via non-oxidative metabolism of ethanol. PEth was measured in femoral blood from 85 consecutive forensic autopsies and was detected in 35 of the cases at concentrations ranging from 0.8 to 22.0 micromol/l. Of the PEth positive cases, 12 did not have significant levels of ethanol in the blood. Two cases (both suicides involving hanging) had detectable ethanol, but no PEth present in the blood. We conclude that measurements of PEth provide indications of previous alcohol abuse in cases where this may not otherwise be evident.

Characterization of phosphatidylserine transport to the locus of phosphatidylserine decarboxylase 2 in permeabilized yeast

In yeast, nascent phosphatidylserine (PtdSer) can be transported to the mitochondria and Golgi/vacuole for decarboxylation to synthesize phosphatidylethanolamine (PtdEtn). In strains with a psd1Delta allele for the mitochondrial PtdSer decarboxylase, the conversion of nascent PtdSer to PtdEtn can serve as an indicator of lipid transport to the locus of PtdSer decarboxylase 2 (Psd2p) in the Golgi/vacuole. We have followed the metabolism of [(3)H]serine into PtdSer and PtdEtn to study lipid transport in permeabilized psd1Delta yeast. The permeabilized cells synthesize (3)H-PtdSer and, after a 20-min lag, decarboxylate it to form [(3)H]PtdEtn. Formation of [(3)H]PtdEtn is linear between 20 and 100 min of incubation and does not require ongoing PtdSer synthesis. PtdSer transport can be resolved into a two-component system using washed, permeabilized psd1Delta cells as donors and membranes isolated by ultracentrifugation as acceptors. With this system, the transport-dependent decarboxylation of nascent PtdSer is dependent upon the concentration of acceptor membranes, requires Mn(2+) but not nucleotides, and is inhibited by EDTA. High speed membranes isolated from a previously identified PtdSer transport mutant, pstB2, contain normal Psd2p activity but fail to reconstitute PtdSer transport and decarboxylation. Reconstitution with permutations of wild type and pstB2Delta donors and acceptors identifies the site of the mutant defect as the acceptor side of the transport reaction.

Differential activation of phospholipase D by VPAC and PAC1 receptors

To investigate the phospholipase D (PLD) responses of the VIP/PACAP receptors, VPAC1 and VPAC2, and the PACAP-specific PAC1 receptors (short and "hop" intracellular loop 3 (i3) splice variants), stable CHO cell lines expressing similar levels of each wildtype receptor were generated (except for the VPAC1 receptor clone which showed considerably lower expression and lesser responses in signalling assays). All clones caused activation of PLD in response to agonists, as monitored by [3H]phosphatidylbutanol production. The PLD responses of the PAC1 "hop", but not the "null" receptor, were sensitive to the ARF inhibitor, brefeldin A (BFA) (as were VPAC1 and VPAC2 responses). Chimeric constructs of VPAC2 receptors containing i3 of either PAC1 hop or PAC1 null receptors were transiently expressed in COS 7 cells and PLD responses were measured. Only the PLD response of the hop construct was sensitive to BFA. This suggests that i3 motifs in certain Group II GPCRs may play a key role in determining their linkage to ARF-dependent PLD activation.

Metabolic fate of [14C]-ethanol into endothelial cell phospholipids including platelet-activating factor, sphingomyelin and phosphatidylethanol

The metabolic fate of ethanol into the phospholipid pool of calf pulmonary artery endothelial cells was studied. [14C]-ethanol was incorporated into various endothelial cell phospholipids including phosphatidylethanol (PEth), which may represent a substantial fraction in microdomains of membrane phospholipids. The incorporation into phospholipids was reduced in the presence of pyrazole and cyanamide, inhibitors of ethanol metabolism. Wortmannin, the phosphatidylinositol 3-kinase inhibitor, increased [14C]-PEth formation. [3H]-acetate was also incorporated into endothelial cell phospholipids but in a different pattern. Distribution of [3H]-acetate and [14C]-ethanol into the fatty acyl moiety versus the glycerophosphoryl backbone of the phospholipids was also different. Stimulation of the endothelial cells with ATP increased [3H]-acetate incorporation into platelet-activating factor (PAF) and ethanol decreased it. Ethanol exposure increased ATP-stimulated [3H]-acetate incorporation into sphingomyelin. However, ATP had no effect on the incorporation of [14C]-ethanol into any phospholipids. The results suggest that the two precursors contribute to a separate acetate pool and that the sphingomyelin cycle may be sensitized in ethanol-treated cells. Thus, metabolic conversions of ethanol into lipids and the effect of ethanol on specific lipid mediators, e.g PAF, PEth and sphingomyelin, may be critical determinants in the altered responses of the endothelium in alcoholism.

Effect of ethanol on ATP-induced phospholipases C and D and serine base exchange in glioma C6 cells

The effect of extracellular ATP, a nucleotide receptor agonist in the central nervous system, was investigated in glioma C6 cells on the intracellular Ca2+ level and the formation of phosphatidylethanol and phosphatidic acid in the presence and absence of ethanol (150 mM). In the cells prelabeled with [14C]palmitic acid, 100 microM ATP induced both the hydrolysis and the transphosphatidylation reactions leading to the formation of [14C]phosphatidic acid; addition of ethanol generated [14C]phosphatidylethanol. However, ATP-mediated increase in the level of [14C]phosphatidic acid was not inhibited by ethanol. Furthermore, ethanol augmented ATP-induced transient and sustained increase in the intracellular Ca2+ concentration, whereas ethanol alone did not produce any change in the intracellular Ca2+ level. These results indicate that in glioma C6 cells, ATP induces activation of polyphosphoinositide-specific phospholipase C and phospholipase D and that ethanol enhances this effect. In the present investigation we have also shown that long-term (2 days) ethanol treatment, at concentration relevant to chronic alcoholism (100 mM), decreased the incorporation of [14C]serine into phosphatidylserine. Since the effect of ethanol on ATP-induced activities of phospholipase C and phospholipase D and on serine base-exchange in glioma C6 cells differs significantly from that in cultured neuronal cells, these results may contribute to a better understanding of the mechanisms of ethanol action in cells of glial origin.

(2R,1'S,2'R,3'S)-2-(2'-Carboxy-3'-phenylcyclopropyl)glycine (PCCG-13), the first potent and selective competitive antagonist of phospholipase D-coupled metabotropic glutamate receptors: asymmetric synthesis and preliminary biological properties

The asymmetric synthesis of (2R,1'S,2'R, 3'S)-2-(2'-carboxy-3'-phenylcyclopropyl)glycine (PCCG-13), a trisubstituted carboxycyclopropylglycine endowed with unusual stereochemical features, is described. Preliminary biological evaluation demonstrates PCCG-13 as a very potent and selective competitive antagonist for the novel class of metabotropic glutamate (mGlu) receptors coupled to the activity of phospholipase D (PLD). PCCG-13 is therefore a useful tool for the exploration of the physiopathological role of this novel class of receptors.

Thromboxane A2-induced phosphatidylcholine hydrolysis in porcine vascular smooth muscle cells

The effect of 9,11-epithio-11,12-methanothromboxane A2 (STA2), a thromboxane A2 receptor agonist, on phosphatidylcholine hydrolysis was examined in porcine vascular smooth muscle cells. Although STA2 stimulated diacylglycerol production in a concentration-dependent manner, it only caused a slight accumulation of [3H]phosphatidylethanol in the presence of 0.5% ethanol, reflecting its weak stimulation of phosphatidylcholine-specific phospholipase D. STA2-induced diacylglycerol production was potently and concentration dependently inhibited by potassium tricyclo-[5.2.1.0(2.6)]-decyl-(9[8])-xanthogenate (D609), an inhibitor of phosphatidylcholine-specific phospholipase C. These results suggest that the thromboxane A2 receptor in vascular smooth muscles is functionally coupled to phosphatidylcholine-specific phospholipase C to yield diacylglycerol.

Involvement of lipid mediators on cytokine signaling and induction of secretory phospholipase A2 in immortalized astrocytes (DITNC)

Our previous studies demonstrated the ability of proinflammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha) and interleukin 1beta (IL-1beta), to stimulate NFkappaB/DNA binding and synthesis of secretory phospholipase A2 (sPLA2) in immortalized astrocytes (DITNC). In this study, we examined possible involvement of lipid mediators in the cytokine action. Using [14C]serine to label sphingomyelin and ceramide in these cells, subsequent exposure of cells to cytokines did not result in alteration of sphingomyelin/ceramide ratio. Furthermore, neither exogenous sphingomyelinase nor cell-permeable ceramides could stimulate NFkappaB/DNA binding. On the other hand, C-2 ceramide (0.3 microM) as well as other lipid mediators, such as lysophosphatidylcholine and arachidonic acid, were able to elicit a small increase in sPLA2 and potentiate the induction of sPLA2 by TNF-alpha. When DITNC cells were prelabeled with [32P]Pi, an increase in labeled phosphatidic acid (PA) was observed on treatment of cells with IL-1beta (200 U/mL). However, despite the ability of phorbol myristate acetate (PMA) to stimulate phospholipase D (PLD) and synthesis of phosphatidylethanol (PEt) in these cells, PLD activity was not affected by IL-1beta. With the [32P]labeled cells, however, PA-phosphohydrolase inhibitors, such as chlorpromazine and propranolol, could elicit large increases in labeled PA, indicating active PA metabolism in these cells. Cytokines also caused an increase in levels of diacylglycerol (DG) in these cells, although the source of this lipid pool is presently not understood. Taken together, these results provide evidence for the participation of PA and DG in cytokine signaling activity. Furthermore, although cytokines did not cause the release of ceramide, lipid mediators, such as lysophospholipids, and AA could modulate cytokine-mediated induction of sPLA2 in astrocytes.

Formation of phosphatidic acid and subclasses of phosphatidylethanol in human neutrophils upon interleukin-8 stimulation

Previous studies showed that interleukin-8 (IL-8) stimulates phospholipase D hydrolysis of phosphatidylcholine to generate phosphatidic acid in human neutrophils. Phosphatidylcholine in these cells contains diacyl, alkylacyl and alkenylacyl subclasses. No studies have examined phospholipase D hydrolysis of the three subclasses of phosphatidylcholine in interleukin-8-stimulated neutrophils. We used a non-radioactive but very sensitive method to assess the relative distribution of the subclasses in phosphatidylethanol, which is derived from phospholipase D activity in ethanol-exposed neutrophils. We present evidence that the relative abundance of diacyl and alkylacyl subclasses in phosphatidylethanol is similar to that in phosphatidylcholine. Alkenylacyl subclass was also detectable in the phosphatidylethanol fraction, albeit as a minor subclass. Our findings suggest that phospholipase D catalyses the hydrolysis of diacyl, alkylacyl and alkenylacyl subclasses of phosphatidylcholine in neutrophils upon IL-8 stimulation.

Possible involvement of mitogen-activated protein kinase in phospholipase D activation induced by H2O2, but not by carbachol, in rat pheochromocytoma PC12 cells

We have previously reported that hydrogen peroxide (H2O2) induced a considerable increase of phospholipase D (PLD) activity and phosphorylation of mitogen-activated protein (MAP) kinase in PC12 cells. H2O2-induced PLD activation and MAP kinase phosphorylation were dose-dependently inhibited by a specific MAP kinase kinase inhibitor, PD 098059. In contrast, carbachol-mediated PLD activation was not inhibited by the PD 098059 pretreatment whereas MAP kinase phosphorylation was prevented. These findings indicated that MAP kinase is implicated in the PLD activation induced by H2O2, but not by carbachol. In the present study, H2O2 also caused a marked release of oleic acid (OA) from membrane phospholipids in PC12 cells. As we have previously shown that OA stimulates PLD activity in PC12 cells, the mechanism of H2O2-induced fatty acid liberation and its relation to PLD activation were investigated. Pretreatment of the cells with methylarachidonyl fluorophosphonate (MAFP), a phospholipase A2 (PLA2) inhibitor, almost completely prevented the release of [3H]OA by H2O2 treatment. From the preferential release of OA and sensitivity to other PLA2 inhibitors, the involvement of a Ca2+-independent cytosolic PLA2-type enzyme was suggested. In contrast to OA release, MAFP did not inhibit PLD activation by H2O2. The inhibitory profile of the OA release by PD 098059 did not show any correlation with that of MAP kinase. These results lead us to suggest that H2O2-induced PLD activation may be mediated by MAP kinase and also that H2O2-mediated OA release, which would be catalyzed by a Ca2+-independent cytosolic PLA2-like enzyme, is not linked to the PLD activation in PC12 cells.

Increased phospholipase D activity in butyrate-induced differentiation of HT-29 cells

Phospholipids are important constituents of biomembrane components and are supposed to function as enzyme activators or precursors of bioactive substances. Our earlier work has shown an increased esterification of neutral lipids of HT-29 cells during butyrate-induced differentiation (M. Madesh, O. Benard, K.A. Balasubramanian, Butyrate-induced alteration in lipid composition of human colon cell line HT-29, Biochem. Mol. Biol. Int. 38 (1996) 659-664). In this report we show that there is an increase in phospholipase D (PLD) activity during butyrate-induced differentiation of HT-29 cells as indicated by the formation of phosphatidic acid (PA). When the control and butyrate-treated cell homogenates were incubated in vitro with 1 mM Ca2+, the increase in PA formation was higher than in butyrate-treated cells. This PA was formed due to PLD activity that was confirmed by the generation of phosphatidylethanol by in vitro incubation of HT-29 cell homogenates in the presence of ethanol. The formation of PA was associated with a decrease in phosphatidylcholine (PC) and phosphatidylethanolamine (PE). This study has shown an increase in PLD activity associated with the differentiation of HT-29 cells.

Increased levels of methylated intermediates of phosphatidylcholine lead to enhanced phospholipase D activity

Previous work from this laboratory and others has shown that neurotransmitters can activate phospholipase D. Unlike the phospholipase C that specifically hydrolyzes inositol-containing phospholipids, phospholipase D in neuronal tissue specifically hydrolyzes phosphatidylcholine. One route for the synthesis of phosphatidylcholine, is via methylation of phosphatidylethanolamine. Using an in vitro assay, we have previously shown that methylated intermediates are also good substrates for phospholipase D (1). In this manuscript we demonstrate that these intermediates are also substrates in the intact PC12 cells. Cells incubated with methyl and dimethylethanolamine incorporate more [3H]palmitic acid into the corresponding phospholipid, phosphatidyl-N-methylethanolamine and phosphatidyl-N,N-dimethylethanolamine. In these cells bradykinin causes a greater increase in [3H]phosphatidylethanol production. Elevated levels of [3H]phosphatidylcholine do not enhance bradykinin-stimulated [3H]phosphatidylethanol production, therefore, this effect is specific for the methylated intermediates. Finally, this effect is not due to some generalized enhancement of receptor coupling because incubation of the cells with methylethanolamine does not lead to an increase in bradykinin stimulated inositol phosphate production.

GnRH signalling pathways and GnRH-induced homologous desensitization in a gonadotrope cell line (alphaT3-1)

Exposure of the gonadotrope cells to gonadotropin-releasing hormone (GnRH) reduces their responsiveness to a new GnRH stimulation (homologous desensitization). The time frame as well as the mechanisms underlying this phenomenon are yet unclear. We studied in a gonadotrope cell line (alphaT3-1) the effects of short as well as long term GnRH pretreatments on the GnRH-induced phospholipases-C (PLC), -A2 (PLA2) and -D (PLD) activities, by measuring the production of IP3, total inositol phosphates (IPs), arachidonic acid (AA) and phosphatidylethanol (PEt) respectively. We demonstrated that although rapid desensitization of GnRH-induced IP3 formation did not occur in these cells, persistent stimulation of cells with GnRH or its analogue resulted in a time-dependent attenuation of GnRH-elicited IPs formation. GnRH-induced IPs desensitization was potentiated after direct activation of PKC by the phorbol ester TPA, suggesting the involvement of distinct mechanisms in the uncoupling exerted by either GnRH or TPA on GnRH-stimulated PI hydrolysis. The levels of individual phosphoinositides remained unchanged under any desensitization condition applied. Interestingly, while the GnRH-induced PLA2 activity was rapidly desensitized (2.5 min) after GnRH pretreatments, the neuropeptide-evoked PLD activation was affected at later times, indicating an important time-dependent contribution of these enzymatic activities in the sequential events underlying the GnRH-induced homologous desensitization processes in the gonadotropes. Under GnRH desensitization conditions, TPA was still able to induce PLD activation and to further potentiate the GnRH-evoked PLD activity. AlphaT3-1 cells possess several PKC isoforms which, except PKCzeta, were differentially down-regulated by TPA (PKCalpha, betaII, delta, epsilon, eta) or GnRH (PKCbetaII, delta, epsilon, eta). In spite of the presence of PKC inhibitors or down-regulation of PKC isoforms by TPA, the desensitizing effect of the neuropeptide on GnRH-induced IPs, AA and PEt formation remained unchanged. In conclusion, in alphaT3-1 cells the GnRH-induced homologous desensitization affects the GnRH coupling with PLC, PLA2 and PLD by mechanism(s) which do not implicate TPA-sensitive PKC isoforms, but likely reflect time-dependent modification(s) on the activation processes of the enzymes.

Angiotensin II-mediated stimulation of phospholipase D in rabbit kidney proximal tubule cells

The present study was undertaken to demonstrate whether or not angiotensin II activates a phospholipase D in rabbit kidney proximal tubule cells. By measuring the formation of [3H]phosphatidic acid and [3H]phosphatidylethanol, we elucidate the direct stimulation of phospholipase D by angiotensin II. Angiotensin II leads to a rapid increase in [3H]phosphatidic acid and [3H]diacylglycerol, and [3H]phosphatidic acid formation preceded the formation of [3H]diacylglycerol. This result suggests that some phosphatidic acid seems to be formed directly from phosphatidylcholine by the action of phospholipase D, not from the action of diacylglycerol kinase on the diacylglycerol. In addition, the other mechanisms by which phospholipase D is activated was examined. We have found that phospholipase D was activated by extracellular calcium ion. It has also been shown that angiotensin II may activate phospholipase D through protein kinase C-independent pathway.